1. Field of the Invention
This invention relates to the in-vivo determination and display of estimates of the cardiac ejection fraction, or the end diastolic volume, or both.
2. Description of the Related Art
Information about the output of a patient's heart is very valuable to a surgical team operating on the patient or to physicians who are trying to diagnose an illness or monitor the patient's condition. Few hospitals are therefore without some form of conventional equipment to monitor cardiac output.
One common way to determine cardiac output is to mount some flow-measuring devices on a catheter, and then to thread the catheter into the patient and to maneuver it so that the devices are in or near the patient's heart. Some such devices inject either a bolus or heat at an upstream position, such as in the right atrium, and determine flow based on the characteristics of the injected material or energy at a downstream position, such as in the pulmonary artery.
For example, U.S. Pat. No. 4,236,527 (Newbower et al., Dec. 2, 1980) and U.S. Pat. No. 4,507,974 (Yelderman, Apr. 2, 1985), describe systems for measuring cardiac output in which heat is used as an indicator. In such heat-based systems, a balloon catheter is typically positioned proximal to the branch of the pulmonary artery via the right atrium and the right ventricle. The catheter includes a resistive heating element, which is positioned in the atrium and/or ventricle, and a thermistor, which is positioned in the artery. Cardiac output is then calculated as a function of the sensed downstream temperature profile.
U.S. Pat. No. 5,146,414 (McKown, et al., Sep. 8, 1992) describes a system in which the transfer function of the channel (the region from where an indicator such as heat is applied to the blood upstream to the downstream position where the indicator concentration, such as temperature, is sensed) is modeled, the approximate spectrum of the noise is determined, and the output of the system is used in a feed-back loop to adaptively update the parameters of the model and thus to improve the estimate of cardiac output (CO). U.S. Pat. No. 5,687,733 (McKown, et al., Nov. 18, 1997) describes an improvement over the earlier McKown '414 system that estimates both the CO trend and an instantaneous CO value. Moreover, in the McKown systems, only the zero-frequency (dc or steady state) gain of the channel is required to get an estimate of the cardiac output (CO).
Although these known systems provide estimates of cardiac output with varying degrees of accuracy, they fail to provide any estimate of the heart's ejection fraction (EF), which is defined as the ratio between the stroke volume (SV) of the heart and its end diastolic volume (EDV). The ejection fraction is thus a measure of how efficiently the heart pumps out the blood that it can contain.
Because of its diagnostic importance, there are several known methods for measuring EF. Such systems, however, frequently rely on the use of an injected bolus and on evaluation of the wash-out (thermodilution) curve in the blood vessel. U.S. Pat. No. 4,858,618 (Konno, et al., issued Aug. 22, 1989), for example, describes a thermodilution system for determining right ventricular ejection fraction. In this known system, a cold bolus indicator is injected into the right ventricle. Pre- and post-bolus temperatures and sensed in the pulmonary artery. The temperature differentials are used to determine the ejection fraction.
One problem with using a bolus to determine EF is that it is difficult to establish just where on the sensed bolus curve the measurements are to begin, since the front side of the curve depends heavily on mixing, on the heart rate, and even on how fast the administering nurse is pushing the syringe plunger while injecting the bolus. Another problem faced by all such known systems is that they require synchronization with the heart cycle in order to reduce the effects of the heartbeat when producing an EF estimate. Some systems synchronize based on plateaus in the wash-out curve, but this presupposes a fast and very accurate thermistor. Other systems rely for synchronization on an EKG trigger. EKG synchronization, however, is difficult, since it is then necessary to slave in and precisely coordinate the timing of other instruments, each gathering its own data.
Further problems of existing systems for determining EF stem from their need to identify discrete plateaus in the dilution profiles created by the heart beats. This is necessary because these systems use the plateaus as markers in order to fit exponential or ratio-based curves to the data, which are in turn used to evaluate the dilution decay. This approach is accurate in practice, however, only for a relatively slow heart rate and a thermistor whose response is significantly faster than the decay parameter .tau..
In effect, these conventional systems assume a square-wave dilution curve. This is, however, usually an unrealistic assumption. First, most of the patients needing EF measurements in a hospital are not in the best of health; rather, they tend to have relatively high and erratic heart rates. Furthermore, in systems that use a bolus of relatively cold fluid, the sensed heart rate is likely to be incorrect since the cold bolus itself tends to affect not only the heart rate, but also its regularity. Second, real thermistors distort the plateaus, so that the exponential fits themselves become distorted. Third, as the EF rises, the drops in the plateaus also rise. This causes the systems to use fewer plateaus, and thus reduce their accuracy, because of the limited signal-to-noise ratios of these systems.
What is needed is therefore a system that can produce continuous estimates of the EF or EDV, or both. This invention provides such a system.